1. Field of the Invention
This invention relates to liquid pumping systems and, more particularly, to constant-pressure liquid spraying systems for use as paint sprayers or similar applications.
2. Description of the Related Art
Spraying systems have supplanted older, labor intensive liquid delivery systems for many applications. The construction industry in particular has seen a significant increase in the use of spraying systems for applying liquid materials to structural surfaces. For example, stucco, drywall "texture" material, insulation/fire retardant materials and paint, which at one time were applied almost exclusively with a trowel, roller, or brush, are now often sprayed onto a target surface. Because painting is probably the most widely-used of these applications, the following discussion will refer to paint spraying, but the problems and solutions apply to all of the above-mentioned applications.
Paint spraying systems typically consist of a reservoir, hoses, pump, pump motor, pump motor controller and "spray gun". The reservoir holds the paint, hoses (or pipes) deliver the paint to the pump and the pump is operated by the motor. Another hose delivers the paint from the pump to the spray gun, where a painter controls the flow of paint by operating a trigger on the gun. Typically, the trigger provides "ON/OFF" control, i.e., when depressed the trigger permits the flow of paint from the hose at a rate which is largely determined by the pressure of the pump and the restriction of the hoses and spray gun orifice (spray tip). When released, the trigger shuts off the flow of paint by closing a valve or "shutter" within the gun.
When operating the paint sprayer, a painter will move along a target surface, e.g. a wall, spraying a portion of the wall with each sweep (horizontal or vertical) of the spray gun. Ideally the pump pressure remains constant as the painter moves along the wall, spraying adjacent sections of the wall with each sweep and applying an even coat of paint to the wall.
However, if the pump pressure does not remain constant, the paint can be applied unevenly. Each painted section preferably has a relatively straight border so that the adjacent section may be painted using a relatively straight motion without creating sections of excessive overlap and/or areas devoid of paint. But, if the pump pressure varies while a section is painted, the spray pattern width will also vary, making it difficult to properly overlap adjacent areas of paint. In addition, pressure variations may produce uneven atomization of the paint, resulting in an uneven thickness of the paint coat. These problems can be very noticeable.
Furthermore, it is desirable to avoid over-spray in any case. Painters typically "mask off" an area that is not to be painted. Precise control of the spraying system's pressure would provide more exact control of the system's spray pattern and may eliminate some of the time consuming masking operation.
Some applications require greater precision than others. Painting the trim on a house, for example, requires greater precision and control than painting a 10 meter by 40 meter warehouse wall. When painting the wall with elastomeric or latex paint, a painter may use the highest pressure setting available on the paint sprayer in order to achieve rapid coverage and a uniform spray pattern. Conversely, when painting the house trim with stain, the painter would set the sprayer at a much lower setting to provide good atomization. At low pressure, when spraying stain for example, pressure variations have a greater impact upon the sprayer's spray pattern. For these reasons, uniformity of pressure is even more important at low pressure settings than at high pressures.
In one approach, a painter sets the pump pressure to a desired level by adjusting a control input such as a dial on the spray system. The system's output pressure is sensed using a resistive strain gauge bridge, and the differential voltage from the bridge is fed to a differential amplifier which provides a signal representative of the system's measured output pressure. This signal is compared with one which represents the desired pressure setting, i.e. the dial setting. The result of this comparison, an error signal, is used to control speed of the pump motor by turning the motor if the pressure is too high, or by increasing the speed if the pressure is too low.
One of the problems with this approach to controlling pressure is that at low pressure levels the pressure sensor is basically using the same scale as at higher pressures; thus system pressure tolerances are approximately the same throughout the sprayer's pressure range. A 344.756 kPa (50 psi) error at a pressure setting of 20.685 MPa (3000 psi) will create the same error signal and same response from the control system as a 344.756 kPa (50 psi) error at a pressure setting of 2.068 MPa (300 psi). This indifference to scale has undesirable consequences in practice. For example, a spray system may provide a pressure range of 2.068 MPa to 20.685 MPa (300 to 3000 psi). If the pump produces 21.030 MPa(3050 psi) instead of a desired 20.685 MPa (3000 psi), the spray pattern out of the spray gun will be slightly wider than desired. If, using the same tolerances, the pump produces 2.413 MPa (350 psi) instead of 2.068 MPa (300 psi), the spray pattern out of the spray gun will be wider by a similar absolute amount.
Although the width of the spray pattern is "off" by the same amount in the preceding example, the resulting pattern error could have more serious consequences in a low-pressure, precision painting application than in a high-pressure application. Additionally, variations in the system's dynamic output pressure (the inevitable fluctuations which occur while pumping) will similarly have more serious consequences at lower pressures.
Another problem with paint sprayers which employ conventional control systems is that they tend to exhibit static pressures which are substantially higher than their dynamic pressures. That is, for a given dial pressure the sprayer's output pressure is much higher when no paint is being sprayed (the trigger is released) than when paint is being sprayed. Thus, the spray pattern is much broader, and there is substantial over-spray when the trigger is initially depressed compared to when the spray pattern has shrunk to the desired size once the control loop stabilizes.
Generally, while liquid is being sprayed the sprayer's output pressure drops until it reaches a "turn on" set point, at which time the system's controller turns the pump motor on. With the pump motor on, the output pressure rises until it reaches a "turn off" set point at which the motor is turned off. Due to a lag in the control system, the pump continues to run for a period of time after the "turn off" threshold is reached. If the sprayer's trigger has been released, the system's output pressure continues to increase substantially because, although the pump continues to operate, there is no flow of paint out of the sprayer. This is the main mechanism for creating the difference between static and dynamic pump pressures.
For the forgoing reasons there is a need for a liquid spraying system which provides a more uniform dynamic pressure, especially at the low pressure range of the spraying system, and which reduces the difference between the system's dynamic and static pressures. There is also a need for a system which achieves these goals using a low complexity feedback-control-loop.